Ring-Opening of 1,3-Imidazole Based Mesoionic Carbenes (iMICs) and Ring-Closing Clicks: Facile Access to iMIC-Compounds

Herein, ring-opening of mesoionic carbenes (iMICs) (iMIC=[ArC{N(Dipp)}2 C(SiMe3 )C:) (Dipp=2,6-iPr2 C6 H3 , Ar=Ph, 4-Me2 NC6 H4 or 4-PhC6 H4 ) based on an 1,3-imidazole scaffold to yield N-ethynylformimidamide (eFIM) derivatives as crystalline solids (eFIM={(Dipp)N=C(Ar)N(Dipp)}C≡CSiMe3 ) is reported. eFIMs are thermally stable under inert gas atmosphere and show moderate air stability (t1/2= 3 h for Ar=Ph). eFIMs are excellent surrogates of iMICs, which generally have a limited shelf-life, and readily undergo ring-closing click reactions with a variety of main-group as well as transition metal Lewis acids to form hitherto challenging iMIC-compounds in good to excellent yields. In addition to the relevance of eFIMs in the synthesis of iMIC-compounds, quantification of the stereoelectronic properties of a representative iMIC (Ar=Ph) by experimental and theoretical methods suggests remarkably σ-donor property and steric profile of these new ligand sets.

A Density Functional Theory Study on Et-BAC-Catalyzed 1,6-Conjugate Addition of p-Chlorobenzaldehyde to p-Quinone Methide for the Synthesis of α,α'-Diarylated Ketones

Umpolung-based organocatalysis has made a remarkable breakthrough in the field of synthetic organic chemistry. Among a plethora of umpolung catalysts, bis(amino)cyclopropenylidenes (BACs) have emerged as efficient organocatalysts with potential applications in synthesizing numerous essential organic moieties. In this study, a plausible mechanism for bis(diethylamino)cyclopropenylidene (Et-BAC)-catalyzed synthesis of α,α'-diarylated ketones has been established using the density functional theory (DFT) method. The proposed catalytic cycle of the studied reaction initiates with the nucleophilic interaction of Et-BAC with p-chlorobenzaldehyde to form a zwitterionic intermediate, which is then transformed to a reactive Breslow intermediate. The Breslow intermediate further undergoes a chemoselective and stereoselective 1,6-conjugate addition reaction with p-quinone methide to form a new C-C bond connection. Finally, the generated adduct undergoes a proton shift reaction with the assistance of both 8-diazabicyclo(5.4.0)undec-7-ene (DBU) and protonated DBU to yield the desired product. Conceptual DFT-derived reactivity indices and frontier molecular orbital theory analysis have been successfully utilized to unravel the role of Et-BAC in this studied reaction. In addition to Et-BAC, DBU and protonated DBU also play a very important role in lowering the activation energy barrier of proton transfer steps. This investigation will help in the rational designing of simple nonheterocyclic carbene-mediated novel organic transformations.

An N-heterocyclic carbene-catalyzed switchable reaction of 9-(trimethylsilyl)fluorene and aldehydes: chemoselective synthesis of dibenzofulvenes and fluorenyl alcohols

An N-heterocyclic carbene-catalyzed synthesis of dibenzofulvenes and fluorenyl alcohols was developed. In the presence of 10 mol% NHC (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and 4 Å molecular sieves, 9-(trimethylsilyl)fluorene undergoes an olefination reaction with aldehydes to produce dibenzofulvenes in 43-99% yields. However, on reducing the NHC loading to 1 mol% and with the addition of water, 9-(trimethylsilyl)fluorene selectively undergoes nucleophilic addition with aldehydes to afford fluorenyl alcohols in 40-95% yields.

Halogen-containing heteroaromatic carbenes of the 1,2,4-triazole series and their transformations

A series of new stable halogenated carbenes, 1-tert-butyl-3,4-diaryl-1,2,4-triazol-5-ylidenes, has been synthesized.

Stretching the P-C Bond. Variations on Carbenes and Phosphanes

The stability and the structure of adducts formed between four substituted phosphanes (PX3, X:H, F, Cl, and NMe2) and 11 different carbenes have been investigated by DFT calculations. In most cases, the structure of the adducts depends strongly on the stability of the carbene itself, exhibiting a linear correlation with the increasing dissociation energy of the adduct. Carbenes of low stability form phosphorus ylides (F), which can be described as phosphane → carbene adducts supported with some back-bonding. The most stable carbenes, which have high energy lone pair, do not form stable F-type structures but carbene → phosphane adducts (E-type structure), utilizing the low-lying lowest unoccupied molecular orbital (LUMO) of the phosphane (with electronegative substituents), benefiting also from the carbene-pnictogen interaction. Especially noteworthy is the case of PCl3, which has an extremely low energy LUMO in its T-shaped form. Although this PCl3 structure is a transition state of rather high energy, the large stabilization energy of the complex makes this carbene-phosphane adduct stable. Most interestingly, in case of carbenes with medium stability both F- and E-type structures could be optimized, giving rise to bond-stretch isomerism. Likewise, for phosphorus ylides (F), the stability of the adducts G formed from carbenes with hypovalent phosphorus (PX-phosphinidene) is in a linear relationship with the stabilization of the carbene. Adducts of carbenes with hypervalent phosphorus (PX5) are the most stable when X is electronegative, and the carbene is highly nucleophilic.

N-Heterocyclic Carbenes in Materials Chemistry

N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

Solvent-promoted catalyst-free N-formylation of amines using carbon dioxide under ambient conditions

An unprecedented catalyst-free formylation of amines using CO2 and hydrosilanes was developed. The solvent plays a vital role in promoting the interaction of amines with hydrosilanes and subsequent CO2 insertion, thus facilitating the simultaneous activation of N-H and Si-H bonds. Based on relevant mechanistic studies, a plausible mechanism involving a silyl carbamate intermediate is proposed.

N-Heterocyclic Carbene-Catalysed Mukaiyama–Michael Reaction and Mukaiyama Aldol/Mukaiyama–Michael Three-Component Coupling Reaction

An N-heterocyclic carbene-catalysed Mukaiyama–Michael addition between several trimethylsilyl (TMS) enol ethers and chalcone derivatives has been discovered. In addition, a related reaction cascade involving dehydrative Mukaiyama aldol followed by a Mukaiyama–Michael addition process has been developed. The later reaction can also be achieved as a three-component coupling reaction. The enantioselective variant of these reactions has been examined with homochiral catalysts. Though these catalysts were active, they fail to achieve enantioinduction.

Controlled synthesis of cyclosiloxanes by NHC-catalyzed hydrolytic oxidation of dihydrosilanes

Hydrolytic oxidation of dihydrosilanes with water in acetonitrile with 0.1 mol% of NHC produced hydrogen gas and cyclosiloxanes in excellent yields.

Adduct Formation, B-H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1) with unsaturated and saturated NHCs as well as CAAC(Me) . Mono-NHC adducts of the type HBcat⋅NHC (NHC=nPr2 Im, iPr2 Im, iPr2 Im(Me) , and Dipp2 Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC(Me) with HBcat yielded the B-H activated product CAAC(Me) (H)Bcat via insertion of the carbine-carbon atom into the B-H bond. The saturated NHC Dipp2 SIm reacted in a 2:2 ratio yielding an NHC ring-expanded product at room temperature forming a six-membered -B-C=N-C=C-N- ring via C-N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene-carbon atom.

Unusual Fragmentation and Transformation of an N-Heterocyclic Carbene by a Stable Phosphonium-Borane peri-Functionalized Naphthalene

A 1-phosphonium-8-borane-decorated naphthalene molecule 2 has been found to react with N,N'-dimethylimidazol-2-ylidene (IMe), a popular member of the N-heterocyclic carbene (NHC) family, which converts it into two vinyl-amine fragments one of which is trapped between the phosphonium and borane unit by the formation of a C-C and a B-N bond. The same reactivity was not observed for larger NHC molecules. Control experiments and mechanistic studies have established the involvement of an ylide-borane molecule and an imidazolium salt in addition to IMe carbene in this new transformation of an NHC.

Nucleophilicity of normal and abnormal N-heterocyclic carbenes at DFT: steric effects on tetrazole-5-ylidenes

Nucleophilicity of both normal (1R) and abnormal (2R) N-heterocyclic carbene (NHC); every 2R showing a higher nucleophilicity than its corresponding 1R isomer (R = H, methyl, ethyl, i-propyl, and t-butyl).

The reductive P–P coupling of primary and secondary phosphines mediated by N-heterocyclic carbenes

P–P bond formation! The reaction of an NHC with primary and secondary phosphines leads to dehydrocoupling of the phosphines with the formation of iPr₂ImH₂ and the corresponding carbene-phosphinidene adducts.

Main group catalysed reduction of unsaturated bonds

This Perspective article reviews the recent developments in reduction reactions catalysed by main-group element compounds.

The nature of Pd–carbene and Pd–halogen bonds in (bisNHC)PdX2 type catalysts: insights from density functional theory

Pd–carbene and Pd–halogen play a key role in the catalytic activity of (bisNHC)PdX₂ complexes and they can be fine-tuned with proper substitution in the carbene moiety and choosing a weakly coordinating halide ion.

N-heterocyclic carbene-catalysed pentafluorophenylation of aldehydes

N-heterocyclic carbenes have been utilized as highly efficient organocatalysts to catalyse multifluorophenylation of aldehydes with fluorinated aryltrimethylsilanes to afford the corresponding adducts in 49–99% yields.

Frustrated N-heterocyclic carbene–silylium ion Lewis pairs

The reaction of a sterically demanding NHC affords frustrated carbene-trimethylsilylium ion pairs, which exhibit FLP-type reactivity such as carbon dioxide fixation and metal–halide bond activation.

Organocatalytic direct difluoromethylation of aldehydes and ketones with TMSCF2H

Organocatalytic direct difluoromethylation of aldehydes and ketones with TMSCF₂H.

Syntheses of sterically shielded stable carbenes of the 1,2,4-triazole series and their corresponding palladium complexes: efficient catalysts for chloroarene hydrodechlorination

Sterically shielded 1,2,4-triazol-5-ylidenes and their palladium complexes: catalysts for hydrodechlorination.

NHC catalysed trimethylsilylation of terminal alkynes and indoles with Ruppert's reagent under solvent free conditions

A highly efficient organo-catalytic protocol for the trimethylsilylation of terminal alkynes and N-silylation of indoles employing Ruppert's reagent as a trimethylsilyl source have been developed under solvent and fluoride free conditions.

Exceptionally efficient catalytic hydrodechlorination of persistent organic pollutants: application of new sterically shielded palladium carbene complexes

Exceptionally efficient catalysis of haloarene hydrodehalogenation by palladium carbene complexes.

N-Heterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis

The chemistry of N-heterocyclic carbenes (NHCs) has witnessed tremendous development in the past two decades: NHCs have not only become versatile ligands for transition metals, but have also emerged as powerful organic catalysts in molecular chemistry and, more recently, in metal-free polymer synthesis. To understand the success of NHCs, this review first presents the electronic properties of NHCs, their main synthetic methods, their handling, and their reactivity. Their ability to activate key functional groups (e.g. aldehydes, esters, heterocycles, silyl ketene acetals, alcohols) is then discussed in the context of molecular chemistry. Focus has been placed on the activation of substrates finding analogies with monomers (e.g. bis-aldehydes, multi-isocyanates, cyclic esters, epoxides, N-carboxyanhydrides, etc.) and/or initiators (e.g. hydroxy- or trimethylsilyl-containing reagents) employed in such "organopolymerisation" reactions utilizing NHCs. A variety of metal-free polymers, including aliphatic polyesters and polyethers, poly(α-peptoid)s, poly(meth)acrylates, polyurethanes, or polysiloxanes can be obtained in this way. The last section covers the use of NHCs as structural components of the polymer chain. Indeed, NHC-based photoinitiators, chain transfer agents or functionalizing agents, as well as bifunctional NHC monomer substrates, can also serve for metal-free polymer synthesis.